The present invention relates generally to the field of devices and methods for monitoring batteries. More specifically, the present invention relates to a method for determining at least one characteristic variable for the state of an electrochemical energy storage battery. The present invention also relates to a monitoring device for such a battery.
The wear of energy storage batteries can either be determined by deducing a state or a behavior of the energy storage battery from the operating history, or else by deducing a state or a behavior of the energy storage battery from measurement findings.
DE 195 40 827 C1 discloses a method for determination of aging, in which a family of characteristics for battery aging is evaluated as a function of the amount of discharge per discharge cycle as a battery aging influencing variable. The method takes account of the influencing variables of “discharging” and “discharge depth,” so that the amount of charge discharged is used to determine aging.
Furthermore, U.S. Pat. No. 6,103,408 describes an aging clock whose frequency is varied as a function of at least one characteristic value of the energy storage battery. By way of example, the frequency of the aging clock may be raised when the electrolyte temperature increases and when the discrepancy between the terminal voltage and the rest voltage increases.
Various different methods are known for measurement of the state of charge and for determination of the load response of energy storage batteries. For example, integrating measurement devices are thus used (ampere-hour (Ah) counters), with the charging current being taken into account, possibly weighted with a fixed charging factor. Since the usable capacity of an energy storage battery is highly dependent on the magnitude of the discharge current and the temperature, even a method such as this does not allow a satisfactory conclusion to be drawn about the usable capacity which can still be drawn from the battery.
By way of example, DE 22 42 510 C1 discloses, in the case of a method for measurement of the state of charge, the charging current being weighted with a factor which is dependent on the temperature and on the state of charge of the battery itself.
DE 40 07 883 A1 describes a method in which the starting capability of an energy storage battery is determined by measurement of the battery terminal voltage and of the battery temperature, and by comparison with a family of curves for the state of charge that is applicable to the battery type to be tested.
DE 195 43 874 A1 discloses a calculation method for the discharge characteristic and remaining capacity measurement for an energy storage battery in which current, voltage, and temperature are likewise measured, with the discharge characteristic being approximated by a mathematical function with a curved surface.
DE 39 01 680 C1 describes a method for monitoring the cold-starting capability of a starter battery in which the starter battery is loaded with a resistance at times. The voltage dropped across the resistance is measured and is compared with empirical values in order to determine whether the cold-starting capability of the starter battery is still sufficient. The starting process is in this case used to load the starter battery.
Furthermore, DE 43 39 568 A1 discloses a method for determination of the state of charge of a motor vehicle starter battery in which battery current and rest voltage are measured and the state of charge is deduced from them. In this case, the battery temperature is also taken into account. The charging currents measured during various time periods are compared with one another, and are used to determine the remaining capacity.
DE 198 47 648 A1 describes a method for learning a relationship between the rest voltage and the state of charge of an energy storage battery in order to estimate the storage capability. A measure of the electrolyte capacity of the electrolyte in the energy storage battery is determined from the relationship between the rest voltage difference and the amount of current produced during the loading phase. This makes use of the fact that the rest voltage rises approximately linearly with the state of charge in higher state of charge ranges which are relevant in practice.
One problem in determining the state of an electrochemical energy storage battery using known methods is that wear occurs both while discharging and charging rechargeable energy storage batteries as well as when they are stored without any load applied, and the relevant wear factors are not all taken into account in the process.
In the case of a lead-acid rechargeable battery, the electrolyte is in the form of dilute sulfuric acid, that is to say, a solution of sulfuric acid in water. Typically, this is an approximately 4 to 5 molar solution when in the fully charged state. During the discharge reaction, sulfuric acid is consumed at both electrodes in the electrolyte on the basis of the reaction equation:Positive electrode: PbO2+H2SO4+2H++2e→PbSO4+2H2ONegative electrode: Pb+H2SO4→Pb+2H++2ewith H2O also being formed at a positive electrode. The concentration and the relative density of the electrolyte fall during discharging, while they rise again during the charging reaction, which takes place in the opposite direction.
If the sulfuric acid which is formed during the charging reaction has a convention capability in the earth's field of gravity, then it has the tendency to fall in layers to the bottom of the cell vessel of the cells in the lead-acid rechargeable battery. The electrolyte in the lower area of the respective cell vessel thus has a higher concentration than that in the upper area of the cell vessel. In the case of a lead-acid rechargeable battery, this state is referred to as acid stratification.
Since both the charging/discharge reaction and the parasitic reactions, such as gas development, corrosion etc., are in general influenced by the electrolyte concentration, acid stratification leads to non-uniformity of the state of the cell. However, known methods assess only monotonally developing aging characteristic variables, and do not take account of the effect of stratification of the electrolyte concentration, which can increase and, in certain situations, can also decrease again.
It would be advantageous to provide an improved method for determining at least one characteristic variable for the state of an electrochemical energy storage battery which also takes into account the effect of electrolyte stratification. It would also be advantageous to provide a device (e.g., a monitoring device) for carrying out such a method. It would be desirable to provide a system and/or method that provides any one or more of these or other advantageous features which may be apparent to those reviewing this document.